Cholesteric liquid crystal displays (LCD) have been extensively investigated during the past decade for commercial applications. That solid research has in large arisen from a number of very attractive and potentially profitable applications like portable electronic appliances, including computers and wireless telecommunications devices, electronic books, document viewers, electronic newspapers, portable document assistants etc.
Further, a special attention has been dedicated to the so-called surface stabilized cholesteric texture (SSCT) due to their unique property of bistability and absence of polarizers. Such kind of display mode, if designed to be reflective for short pitch SSCT, in itself is sun readable and consumes no power for information storage. These properties in combination with the technological manufacturing simplicity of SSCT LCDs challenge the existing workhorse of LCD technologies and are considered capable to replace the existing LCD technologies at least in part. SSCT LCD technology is simpler than the conventional twisted nematic (TN) technology and is manufactured by similar manufacturing steps. As the major share of world LCD market is subdivided onto two basic segments: high information content big TFT displays capable of moving images with TV rates and lower information content passive matrix LCDs involving alphanumeric displays, the present invention primarily relates to the last ones. Although, SSCT displays are proven to operate both with TV rates (U.S. Pat. No. 5,661,533) and high number of multiplexing groups (U.S. Pat. No. 5,748,277), the technology for bulk production is immature for today. Conventionally, SSCT operates in the reflective optical mode offering switching between the optically reflective polydomain planar cholesteric texture, optically slightly scattering focal conic texture and transparent electric field induced homeotropic state. The former two optical states as well as their combination in any portion (gray shades) could be stable at zero fields provided special boundary conditions are arranged.
However, there are several major problems to be solved underway to the wide commercialization of SSCT. One of the major problems is insufficiently high voltage for dielectric breakdown of internal black masks in short pitch SSCT LCDs as well as shock sensitivity and mechanical stability of cholesteric texture. The original molecular order of cholesteric texture, if it is stabilized by surface, is easy to destroy just by mechanical deformation when no electric field is applied. This problem is partially solved when cholesteric texture is stabilized in volume by adding certain amount of polymer into cholesteric liquid crystal (LC) composition following subsequent polymerization in display cell (U.S. Pat. Nos. 5,570,216; 5,636,044). Although, the method works, such production of polymer stabilized cholesteric LCDs is hardly fit for mass production. In case of SSCT liquid crystal bulk is influenced mainly by the externally applied electric (magnetic) fields and to a much lower extent by the surface, which is amorphous dielectric in most cases. Both TN/STN and SSCT displays have a sandwich structure and are controlled via the dielectric coupling at the cross section of designated patterned conductive electrodes. Such display design represents alternation of dielectric and conductive sites. An example of a previously known patterning of the electrode in static driven SSCT is e.g. disclosed in WO 2004/021077A1. The size of dielectric sites may be very large, especially for alphanumeric display design. This creates a problem for SSCT since uncontrollable sites may represent artifacts due to shock sensitivity. Unlike TN, where original LC texture uniformity is achieved due to the alignment layers treatment in uncontrolled areas, SSCT would not show any uniformity in electrode free area. As a result, the uncontrollable sites must be covered with a dark opaque mask. A black layer is also normally used behind the sandwich to allow the absorption of all transmitted light. The necessity of such a black mask is at least partly responsible for the drastic display price increase and usually may be performed in at least two different ways: vacuum deposition of dielectrics (for example Ge following subsequent high temperature oxidation to GeO) and spin coating of black polyimide layer following subsequent selective exposure and wet bench processing. Although, the last technique seems to be very simple, it significantly influence the reliability of display operation at high electric fields, due to dielectric breakdown of black polyimides, which is conventionally the case in short pitch SSCT.
There is therefore a need for a liquid crystal device (LCD), and specifically a so-called stabilized cholesteric texture (SSCT) LCD, which is easier and/or more cost-effective to produce, and/or which provides better technical qualities, such as shock resistance and image quality, than heretofore known devices.